1. Field of the Invention
This invention relates to a touch panel disposed at the front of a display for use as a display integrated input switch, and particularly to a glass-glass type touch panel.
2. Description of the Related Art
A well known prior art touch panel of the glass-glass type called resistance film system is disclosed in Japanese Published Patent Applications JA9-146707, 10-48625, and 10-133817.
FIG. 2 illustrates the construction of such the prior art touch panel in which the upper glass side and the lower glass side are shown in an exploded view. As shown, the upper glass substrate 11 is reduced in thickness as compared to the lower glass substrate 12 such that it may be easily deformed upon application of a downward finger pressure. The upper and lower glass substrates 11 and 12 are formed on their opposing surfaces with transparent films 13 and 14, respectively of conducting material.
The transparent conducting films 13 and 14 comprise ITO (Indium Tin Oxide) films or the like to form resistance films and are provided along their opposite edges with electrodes 15a, 15b and 16a, 16b, respectively.
In the illustrated example, the electrodes 16a, 16b provided on the transparent conducting film 14 formed on the lower glass substrate 12 are arranged along the opposite edges in the X direction of the X-Y axes which correspond to two directions orthogonal to each other in a plane parallel to the face of the transparent conducting film 14. On the other hand, the electrodes 15a, 15b provided on the transparent conducting film 13 formed on the upper glass substrate 11 are arranged along the opposite edges in the Y direction of the transparent conducting film 13. The electrodes 15a, 15b are somewhat extended in the X direction beyond one edge of the transparent conducting film 13 to form lead-out ends 15ax, 15bx, as shown in FIG. 2.
Formed on the lower glass substrate 12 are conducting lead-out patterns 17a-17d or voltage application and detection. The lead-out patterns 17a and 17b are connected with the electrodes 16a and 16b, respectively while the lead-out patterns 17c and 17d have their one ends (inner ends) 17cx and 17dx extend so as to oppose to the lead-out ends 15ax, 15bx which are extensions of the electrodes 15a, 15b on the upper glass substrate 11.
As shown, the lead-out patterns 17a-17d extend so as to terminate in their other ends (outer ends) aligned along one side of the lower glass substrate 12 which may be connected with a FPC cable (not shown) or the like for connection with an external circuit.
The electrodes 15a, 15b, 16a, 16b and the lead-out patterns 17a-17d may be formed by print-applying and firing a conducting paste, for which a silver paste is commonly used.
Further, it is to be noted that many dot spacers 18 are formed in an array over the transparent conducting film 14 of the lower glass substrate 12.
The upper glass substrate 11 and the lower glass substrate 12 constructed as described above are integrally bonded together around their peripheries by means of a seal 19 having spacers incorporated therein, with the transparent conducting films 13 and 14 facing each other.
More specifically, the seal 19 comprises spacers of glass fiber or the like incorporated in a matrix of thermosetting resin such as epoxy-based resin and is applied on the lower glass substrate 12 in a frame-like fashion around its outer periphery as shown, on which the upper glass substrate 11 is superposed, followed by pressing and firing the resulting assembly to bond the upper glass substrate 11 and the lower glass substrate 12 together with a predetermined spacing therebetween.
Concurrently with this process, the lead-out ends 15ax, 15bx which are extended ends of the electrodes 15a, 15b on the upper glass substrate 11 and the opposing inner ends 17cx, 17dx of the lead-out patterns 17c, 17d on the lower glass substrate 12 are connected together by a conducting paste.
In addition, a notched portion 19a provided in the frame-like seal 19 is used as an air inlet for injecting air to bulge the central portion of the upper glass substrate 11 (increase the spacing from the lower glass substrate) after bonding the upper and lower glass substrates 11 and 12 together, and once the air has been injected, the notched portion is sealed.
The touch panel constructed as described above is configured such that a depressing operation on the touch panel at any point thereof will bring the transparent conducting films 13 and 14 into contact with each other at the depressed point so that when a voltage is applied to the electrodes 16a, 16b of the transparent conducting film 14 which is a resistance film at the opposite ends thereof, the potential of which is detected through the transparent conducting film 13 to determine the X coordinate of that point while in the same manner, when a voltage is applied to the electrodes 15a, 15b of the transparent conducting film 13 at the opposite ends thereof, the potential of which is detected through the transparent conducting film 14 to determine the Y coordinate of that point. The X and Y coordinates of that depressed point may thus be determined.
Heretofore, however, the well known touch panel having the construction as described above has had the disadvantage that it is quite susceptible to cracks in the relatively thin upper glass substrate 11 when the upper glass substrate 11 and the lower glass substrate 12 are bonded together by a seal 19, resulting in being rejected as defective parts, and hence reduced yields.
As a result of researching into the causes of frequent occurrence of such cracking, the present inventor has reached the conclusion that it may be ascribable to the facts that the conducting lead-out patterns 17a-17d intersect the seal 19 before extending outside to form intersections (overlapping portions) and that rod-like (columnar) glass fibers has heretofore been used as spacers to be admixed into the seal 19. The situation in which the glass cracking incidental to the prior art example illustrated occurs will be specifically explained by exampling numerical values as follows:
Now assume that the spacing between the upper and lower glass substrates 11 and 12 around their periphery (seal joint) is 9 xcexcm. In this instance, glass fibers on the order of 9 xcexcm in diameter (xcfx86) and 30-50 xcexcm in length have heretofore been used as spacers. That is, such glass fibers have been admixed into thermosetting resin such as epoxy-based resin to form a seal 19.
On the other hand, the lead-out patterns 17a-17d are formed of a silver paste, which contains silver particles with particle size of 10 xcexcm or less, say about 6 xcexcm.
FIG. 3 is an illustration showing the manner in which an upper glass substrate 11 is superposed on a lower glass substrate 12 with a seal 19 interposed therebetween, in which that portion of a lead-out pattern 17a intersecting the seal is diagrammatically shown. Specifically, a silver paste having silver particles as described above incorporated therein is print-applied and fired on a lower glass substrate 12 to form lead-out patterns (only a lead-out pattern 17a is shown in FIG. 3) to a thickness of 8-12 xcexcm as measured in a direction perpendicular to the plane of the drawing (X direction in FIG. 2), and then a seal 19 containing therein spacers in the form of glass fibers of 9 xcexcm in diameter (xcfx86) and 30-50 xcexcm in length as described above being applied around the lower glass substrate 12 in parallel to the plane of the drawing (Y direction in FIG. 2) so as to intersect the lead-out patterns, followed by an upper glass substrate 11 being superposed on the lower glass substrate 12 over the seal and the lead-out patterns. In the drawing, the reference numeral 21 indicates the silver particles; 22 the cured resin of the silver paste; 23 the spacers comprising the glass fibers; and 24 the resin matrix of the seal 19.
As is apparent from FIG. 3, due to the rod-like configuration elongated as compared with the diameter, the spacer 23 overlies the silver particles 21 whereby the thickness of that portion of the seal 19 intersecting the lead-out pattern 17a amounts up to about 20 xcexcm whereas the thickness of those portions of the seal 19 not intersecting the lead-out pattern 17a is about 9.5 xcexcm substantially corresponding to the diameter of the spacer 23. It will be appreciated that the difference in the thickness or height between the two different portions may be as great as 11.5 xcexcm, resulting in formation of bumps and dips on the seal. In such condition, when the pressure is applied to the upper glass substrate 11 while the seal 19 is being fired, the pressing forces will be concentrated on the bumps, that is, stress concentration will occur locally on the bumps, leading to a high incidence of cracks in such bumps.
With a view to overcoming the foregoing problem, it is an object of this invention to provide a touch panel which is configured to prevent the occurrence of cracking during the bonding of the upper and lower glass substrates together to thereby significantly improve the manufacturing yield.
In accordance with a first aspect of the present invention, in the touch panel of the type described above, lead-out patterns are formed from a conducting paste containing particles of conducting material, such as silver particles, and spacers incorporated in a seal comprise spherical particles of electrically insulating material in place of rod-like glass fibers of the prior art. The sphere diameter is preferably selected approximately equal to the particle size of the conducting material particles contained in the conducting paste.
In accordance with another aspect of the present invention, the particle sizes of both the conducting material particles and the sphere diameter of the insulating material particles may be in the range of 6-10 xcexcm.
In accordance with yet another aspect of the present invention, the conducting paste may comprise silver paste while the spacers may comprise glass balls.